The present invention is directed to air conditioning system control, and is more particularly directed to a control system which is both temperature and pressure actuated. The invention is particularly suitable for use in air conditioning systems in motor vehicles, and will described in that context, but it should be appreciated that the control system could be used in other air conditioning systems as well.
It is known to use variable capacity compressors in bus air conditioning systems wherein the compressor can be progressively unloaded as engine rpm increases in an attempt to optimize the capacity/power curve. With typical reciprocating compressors, however, unloading is accomplished by removing cylinders from service, and progressive unloading of cylinders results in vibration. Accordingly, many conventional bus air conditioning systems have operated with the compressor either fully loaded or off.
In a typical operating cycle for a combined heating/air conditioning system, two different thermostats will be used to set upper and lower set point temperatures. When the return air temperature is below the lower set point temperature (e.g. 68.degree. F.), the heating system will be activated and the blower will be operated at low speed. When the temperature rises above the lower set point temperature, the heating system will be deactivated but the blower will continue to operate for venting operation. If the return air temperature rises above the upper set point temperature (e.g. 72.degree. F.), the air conditioning system will be activated with the compressor operating at full load and the blower operating at high speed. If the temperature falls below the upper set point temperature, the air conditioning system will continue to operate at full capacity, but the heating system will also be activated to reheat the air after it passes the evaporator. This simultaneous operation of the air conditioning and heating system is for the purpose of obtaining a desired degree of dehumidification and avoidance of clutch cycling. When the temperature falls below the lower set point temperature, the air conditioning system will be disabled and the heating system will again be enabled. However, once the compressor has been activated, it is unlikely that it will shut off, since the heating capacity exceeds the cooling capacity.
Examples of such an air conditioning control system as described above are the automatic preset temperature control system available from Vapor Transportation Systems of Chicago, Ill., and the pneumatic control systems currently used in many GMC busses.
A problem with the typical system described above is that, without the reheating mode of operation described above, an excessive amount of "off" time may elapse between the time at which the compressor is disengaged when the temperature falls below 68.degree. F. and time at which the compressor is re-engaged when the temperature rises above the upper set point temperature of 72.degree. F. During this off cycle, the evaporator coil tends to warm-up significantly, and no further moisture removal is obtained. In fact, the moisture which has collected on the evaporator coil may be reevaporatored. Further, the on/off cycling of the compressor results in an excessive amount of wear and tear on the compressor clutch.
With reheating, on the other hand, the system operates at design capacity at all times with the heating system adding enough heat to keep the temperature above the lower set point. Excessive energy is wasted in having the compressor remove not only the normal heat build-up in the bus but also that added by the heating system.
U.S. Pat. No,. 4,388,048, assigned to the assignee of the present application, discloses an improved variable capacity compressor. Copending and commonly assigned application Ser. No. 421,191 discloses a control system for the compressor in which the compressor is operated at 33%, 66% or 100% load depending on the monitored temperature of the return air. The capacity dictated by the return air temperature can be increased or decreased in accordance with other sensed parameters such as relative humidity, evaporator coil temperature, etc. The disclosures of both U.S. Pat. No. 4,388,048 and U.S. patent application Ser. No. 421,191 are incorporated herein by reference.
While the compressor and control system described in the above references represent a significant improvement in air conditioning systems, "reheat"-type heating/air conditioning systems still operate in the manner described above, with the air conditioning compressor continuing to operate at full load while the heating system is simultaneously operated as the return air temperature decreases from the upper set point temperature to the lower set point temperature.
It would be desirable, therefore, to provide a more effective means for obtaining desired dehumidification at temperatures between the upper and lower set points. It would be further desirable to obtain this dehumidification while decreasing the energy consumption of the system and also minimizing the strain on the compressor clutch.